1. Field of the Invention
The present invention relates to recorders for recording characters, graphs, and the like on a recording medium.
2. Description of the Prior Art
Recorders on the market have a thermal head carrying one or more dot-like heating elements. The head is caused to run in contact with a sheet of heat sensitive recording paper or in contact with a sheet of ordinary recording paper via an ink ribbon coated with ink which is dissolved by heat, and patterns of desired characters, or the like, are formed and printed on the recording paper by supplying electric pulses (heat pulses) of a predetermined width to each heating element at proper times.
As a power supply for supplying pulses to the thermal head of such thermal printer, a dry battery or 100 V AC power source, DC-converted by an adaptor is typically used.
The voltage of the AC power source frequently varies (FIG. 1 shows an example of AC adaptor power supply voltage variation). Accordingly, if the heat pulse width is fixed, variations would occur in the density of printing according to the level variation of the pulses fed to the thermal head. In some cases, the head may be damaged due to application of an excessively large voltage. Accordingly, it is necessary to control the width of the heat pulse to a proper value corresponding to the supply voltage variation. As a pulse width control means, it has been proposed to make a head travelling pulse motor a dummy load, twice energization of that load being first performed each time of head travel in one direction (that is, a single line printing), and the pulse width being determined based on the average value of detected supply voltages, so that a predetermined calorific value is generated at the head.
This method, however, has a drawback in that when, for example, troughs of supply voltages have been detected, voltages lower than the actual voltages are detected with the resultant possibility of head damage.
In addition, according to this method, there is no way of dealing with supply voltage variation that occurs during line printing. Such supply voltage variation occurs typically when the AC adaptor is inserted unexpectedly while the head is driven by a dry battery. In such case, since the supply voltage rises abruptly, when heat pulses of the same width as before are supplied over a predetermined duration of time, the thermal head would be damaged. To prevent abrupt supply voltage variation, it has been proposed that when two power sources namely a dry battery and an AC adaptor, are used typically in a desk top computer with a thermal printer, these power sources are switched by a slide switch. However, since addition of a mechanical part such as a slide switch increases the size of the equipment, and also becomes cost, this approach is not preferred.
The aforementioned facts apply to a variety of printers as well as the thermal printer.